1. Field of the Invention
This invention relates to fuel cells, and more particularly to electrolyte volume control within a fuel cell.
2. Description of the Prior Art
Electrolyte reservoir layers or plates disposed in fuel cells are well known for accommodating volume changes in the electrolyte during fuel cell operation. The following commonly owned U.S. Patents relate to fuel cell reservoir layers: U.S. Pat. Nos. 3,779,811; 3,905,832; 4,035,551; 4,038,463; and 4,064,322. Obviously, these reservoir layers must be made from either hydrophilic material or from material which is treated so as to be hydrophilic in order that they readily retain electrolyte within their pores.
A material commonly suggested for use as a reservoir layer in a phosphoric acid electrolyte fuel cell is carbon fibers bonded together with a resin, such as a phenolic resin, and heated to convert the resin and carbon fibers to graphite. Alternatively, carbon or graphite fibers may be bonded together with pyrolytic graphite by placing a mat of fibers in a decomposable hydrocarbon (e.g., methane) atmosphere under conditions which cause the hydrocarbon to break down into carbon and hydrogen. The carbon (now pyrolytic graphite) deposits on the fibers. Materials of the two foregoing types are available commercially and are referred to in the art as carbon papers.
Several problems have been experienced when using these materials as electrolyte reservoirs. For example, although these materials are initially hydrophilic, when exposed to air and moisture the material becomes hydrophobic over a period of several months. This is the result of a carbon/oxygen specie forming a layer on the exposed surfaces of the material. Thus, these materials have a poor shelf life or may require special storage prior to use.
Even if these reservoir layers are placed into a fuel cell immediately upon fabrication, when disposed on the anode side of the cell the exposure to hydrogen and water vapor at cell operating temperatures also results in the slow formation of a hydrophobic compound on the exposed surfaces of the material. This reduces the effectiveness of the reservoir layer over a period of time and also makes it difficult to replenish the cell with electrolyte if such becomes necessary.